To solve the above problem, there is being widely used the process of eliminating brilliance and improving the drape property by applying an inorganic material exhibiting a dulling effect during the process of polymerizing the polyamide 6 yarn. But, this process is problematic in that, when the input of the inorganic material is set high to 1.5% by weight relative to the weight of the yarn (polymer), the operational ability becomes degraded and the yarn physical properties are deteriorated due to the nonuniform dispersion of the inorganic material.
Therefore, in the prior art, there is a limitation that an inorganic material of greater than 1.5% by weight cannot be contained in a polyamide 6 yarn, and accordingly, there is a limit in attempting to eliminate the metallic brilliance of the yarn or enhance the drape property.
The process of prior art in applying an inorganic material in a yarn, will be described in more detail. A slurry of inorganic material is prepared through a process of wetting an inorganic material with water, a process of grinding the inorganic material condensed in the above process, a concentration correction process, and a sedimentation process. Next, the slurry is applied during the polymerization process of polyamide 6 to produce a full dull polyamide 6 yarn. The inorganic material mainly used is titanium dioxide having an average diameter of 0.3 to 0.4 μm.
In the above process, the degradation of the operational ability and yarn physical properties does not only occur due to the original particle size of the titanium dioxide, but also occurs because the titanium dioxide is rapidly condensed during the wetting process.
In the above process of the prior art, the condensation of titanium dioxide in the wetting process is unavoidable. The problem of titanium dioxide condensation in the wetting process is overcome by a process in which large particles of titanium dioxide are separated before titanium dioxide is applied in the polymerization process of polyamide 6, and thus only small particles are utilized.
Due to this, the yield of the titanium dioxide slurry production process is degraded, the process becomes complicated because the separation process is added, and the particle size of titanium dioxide applied in the polymerization process is nonuniform.
Therefore, in the process of prior art, in a case where titanium dioxide of more than 1.5% by weight, relative to the weight of a yarn (polymer) is applied, there occurs a problem that the pressure of a pack (spinneret) rapidly increases due to large diameter particles of titanium dioxide, and the yarn tension becomes nonuniform due to a nonuniform dispersion of titanium dioxide, thereby making the bending and cutting of yarns disposed directly below the spinneret more serious.
In the prior art, the input of titanium dioxide cannot be set to more than a predetermined level due to the degradation of the operational ability and physical properties, and accordingly the full dull property and drape property of polyamide 6 yarn is difficult to achieve.
Korean Laid-Open Patent No. 1999-60536 discloses a process for preparing a polyamide yarn by a high speed spinning, which produces a polyamide ultrafine yarn having a monofilament fineness of below 1.0 denier by adding titanium dioxide in the step of polyamide polymerization, wherein the portions directly below the spinneret are maintained at a heating atmosphere.
Although the detailed description of the above prior patent describes that titanium dioxide of 1 to 3% by weight is added in the polyamide polymerization step, every examples of the prior patent describes that titanium dioxide of 1.5% by weight is added in the polyamide polymerization step.
This is because, as described above, in a case that titanium dioxide of more than 1.5% by weight is added in the polyamide polymerization step, there occurs a problem that the operational ability is degraded and the yarn physical properties are deteriorated due to a nonuniform dispersion of titanium dioxide.
Moreover, the above prior patent does not suggests concrete means, for example, preparation conditions of the slurry of titanium dioxide, for overcoming the above problems which occur when applying titanium dioxide of more than 1.5% by weight in the polyamide polymerization step.
Subsequently, also in the Korean Laid-Open Patent No. 1999-60536, in the case where the amount of titanium dioxide is over 1.5% by weight, it is inevitable that a degradation of the operational ability and yarn physical properties will occur due to a nonuniform dispersion of the titanium dioxide.
As a prior art for improving the dispersability of titanium dioxide in polyamide, Korean Laid-Open Patent No. 2003-0012336 discloses a process of applying 0.05 to 0.2 parts by weight of an amine based material relative to a caprolactam monomer while using an existing viscosity stabilizer, i.e., acetic acid.
However, the above prior, art has the drawback that it is difficult to prevent recondensation since a contact between acetic acid and titanium dioxide is possible, though the prior art may be effective to prevent the recondensation of titanium dioxide with an increase in repulsive force between titanium dioxide particle surfaces by pH control in the polymerization process. Further, since polymerization is typically carried out at a high temperature of higher than 250° C., in case of an amine based compound, there may occur a problem that its effect is not realized because the polymerization conditions such as a burning point, viscosity, etc. are not satisfied if the number of carbon atoms is below 10.
Additionally, since acid and amines are used in combination, the activity of polymerization may differ according to the input equivalent ratio. This may be a factor in generating a difference in molecular weight of the final polymer or a difference in the terminal groups.
As another prior art, Korean Laid-Open Patent NO. 2003-0034845 discloses a process of applying 0.05 to 0.2 parts by weight of an aromatic amine relative to caprolactam without using acetic acid.
The above process maintains a good dispersability of titanium dioxide in the polymerization process, but a reduction of the zeta potential occurs at some portions in a reactor due to the abundance of the amine, to thereby cause a deterioration in the dispersability of titanium dioxide. In addition, the breadth of change in relative viscosity (RV) before and after melting increases due to an increase in the terminal amine groups, and therefore it is difficult to control the strength of a final product. And, the above process is known to those skilled in the art as a process generally performed mainly for the purpose of improving deep color-dyeing or improving color difference. To complement the physical properties problem, the process of minimizing the breadth of change in relative viscosity (RV) before and after deep color-dyeing and melting by using an amine having no reactivity and being utilizable as a dyeing site is widely used.
Further, the above-described prior art process all aim only at maintaining the dispersability of titanium dioxide in the polymerization process at a slurry level without mentioning detailed techniques for the production of the titanium dioxide slurry. Accordingly, it is important to economically and efficiently produce a slurry of titanium dioxide with a good dispersability.
In addition, in case of generally using a low molecular amine based compound, there is the possibility that polymerization may be accelerated by the reactivity of the amine, and thus the color of the final polymer becomes poor.
Accordingly, it is an object of the present invention to provide a process for allowing a relatively large quantity of titanium dioxide to be contained in a yarn without degrading the operational ability and physical properties of the yarn by dispersing titanium dioxide uniformly in the polymer by preventing the recondensation of titanium dioxide in the wetting process.
It is another object of the present invention to provide a polyamide 6 yarn which is full dull and has a good drape property because it contains a large quantity of titanium dioxide.